Rob Lowe is joining the cast of ABC's Brothers & Sisters as a full-fledged cast member next season. Since Lowe joined the sudser earlier this year, the show has seen its Nielsen results increase, notes Variety; it has recently scored a second-season order from the network. Lowe plays conservative Sen. Robert McCallister, a romantic foil to Calista Flockhart’s character. He was originally slated to guest star in just a handful of episodes. But his first appearance on Brothers & Sisters last November matched the show's series high in adults 18-49, and he ultimately signed on for the full season as a 'special guest star.'